1. Field of the Invention
The present invention relates to an electrode for electrolytic capacitor and a process of producing the same.
2. Prior Art
Electrolytic Capacitors have historically been increased in capacitancexe2x80x94higher and higher capacitance in smaller and smaller packages. Electrolytic capacitors are often used on the secondary side of a smoothing circuit of direct power supply to assist the promptly starting operation of a central processing unit used for computers such as personal computers. Such capacitors are, in particular, required to have excellent high-frequency properties when dealing with the discharging of a large current to the units.
Various improvements have been made in the electrodes, particularly anodes, to be used in such electrolytic capacitors, to meet these requirements. For aluminum electrolytic capacitors, in which usually an anode is formed of aluminum foil subjected to surface area-enlarging treatment by etching, it has been proposed to form finer capillaries in the metal foil by stepping up the etching level. The etched anode metals are anodized to form a dielectric layer on a micropore surface having increased area in the anode metal foil. Thus, the increased surface area of the dielectric layer results in an increased capacitance of the capacitor.
A tantalum capacitor utilizes, as the electrode, a porous body made by sintering fine powder of metal tantalum, which has valve metal function. The porous electrode has micropores in the porous body and can have significantly great specific surface area. By anodizing the metal porous body, the dielectric layer is formed on the inner surface of the micropores, obtaining a large surface area. The provision of capacitance on the entire surface of the micropores enables the total capacitance of the capacitor to be increased.
Capacitor electrodes made from a fine powder of a metal that has valvular function such as tantalum, aluminum and niobium are disclosed in, for example, Japanese Patent Publication No. 63-283012, Japanese Utility Model Publication Nos. 57-138330, 58-187136, and 59-187129. The capacitor electrodes disclosed in these publications are made by planting a lead wire 41 in a sintered porous body 29 as an anode made from fine powder of a metal that has valvular action, as shown in FIG. 8. In addition, the publications quoted above have proposed that a portion of the porous body, at which the lead is embedded, should be worked thin and flat, that the embedded portion of the lead is limited to some length, and that the porous body is limited to a certain degree of flatness at the portion where the lead 16 is embedded in the sintered body.
Now a conventional method commonly employed to manufacture a tantalum capacitor will be described below. A block has been compressed from a metal tantalum fine powder having micropores corresponding to a high specific surface area having a grade of about 30000 xcexcFxc2x7V/g of equivalent capacitance per unit weight, in specified dimensions and is sintered to a porous body for use as an anode. Then a dielectric layer has been formed on the porous body in a known manner by anodizing. At this time, the micropores of the porous body formed by the dielectric layers are filled with an electrolyte of, for example, manganese dioxide. Then, the tantalum capacitor has been completed by attaching a cathode-connecting electrode to the porous body in known manner.
In order to provide a capacitor having higher capacitance, a tantalum capacitor of higher capacitance should have been achieved by sintering a metal tantalum having greater specific surface area of the micropores corresponding to an equivalent capacitance of, for example, 50000 xcexcFxc2x7V/g, to form the similar dimensions of a block as those described above.
Actually, the tantalum capacitor produced using the fine powder of 50000 xcexcFxc2x7V/g did not have a capacitance as high as expected from the equivalent capacitance of a non-sintered metal tantalum capacitor, and the high-frequency characteristic of the capacitor was lowered unsatisfactory. Thus, the tantalum capacitor produced using the fine powder of 50000 xcexcFxc2x7V/g had an undesirable characteristic for carrying a large current.
Such insufficient capacitance of the electrolytic capacitor of the prior art is considered to be caused primarily by the insufficient filling of the electrolyte in the micropores of the porous body to be make the anode. That is, the electrolyte that substantially performs the function of the anode does not sufficiently reach the dielectric layers of the micropores, and therefore the micropores are not fully utilized to provide capacitance.
A second reason is that the porous body has not been provided with sufficient surface area of the electrolyte as a cathode disposed on the body surface to be jointed and covered with the cathode-connection electrode of a internal contact layer, such as silver-containing conductive resin through a graphite layer in direct contact with the cathode. As such, the contact resistance between the electrolyte and the cathode-connection electrode have been increased. Thus, equivalent series resistance of the total conventional capacitor could not decrease, resulting in a poor high-frequency characteristic.
In order to overcome the above problems, it was necessary to fill the electrolyte into the micropores in the porous body and increase outer surface area of the valve metal porous body to connect with the cathode-connection electrode, then to make the equivalent series resistance lower.
An object of the present invention is to provide an anode for use in an electrolytic capacitor that has a high capacitance and excellent high-frequency characteristic, while being capable of carrying a large current.
Another object of the present invention is to provide a process of producing said electrode.
In the anode for an electrolytic capacitor of the present invention, laminated regions of larger-size micropores are established in a porous body of valve metal in order to establish electric current passages to reach fine size micropore regions, which can exhibit high capacitance, thereby, reducing the inner resistance and increasing the capacitance for the electrode.
Further, in the anode for an electrolytic capacitor of the present invention, side surfaces of the porous body are expanded by linear recesses and/or projections, such as grooves or corrugations, formed on the side surfaces of the porous body. The increased area of the outer surface can reduce the contact resistance between the porous body and a cathode-connecting electrode material attached on the outer surface to be connected to a cathode lead.
In this description, an anode for an electrolytic capacitor comprises at least a porous body of a valve metal as an anode of a capacitor, and may include a valve metal substrate for an anodic electrical collector.
To realize such an anode for an electrolytic capacitor in the present invention, the porous body of valve metal may includes a laminate of a plurality of sinter layers having micropores. In this case, the laminated regions of larger-sized micropores are formed more in the vicinity of the interfaces between the adjacent sinter layers than inside each of the sinter layers. The larger micropore-size regions close to the interfaces, when filled with solid electrolyte, establish electric current passages to the inner region of the sinter layers, thereby reducing inner resistance.
Such a laminate in the porous body may be formed by sintering a laminate composed of a plurality of preforms, which are previously formed from a powder of valve metal.
In another aspect of the invention, the larger micropore-size regions and the smaller micropore-size regions may be at least two types of sinter layers, wherein certain properties differ quantitatively.
The larger micropore-size regions may comprise the sinter layers having large micropores, which are made contact with other sinter layers having smaller micropores. The larger micropore-size sinter layers and the smaller micropore-size sinter layers are laminated of preforms having large micropores of a valve metal powder and preforms having small micropores of the same, respectively.
The laminate may include a larger micropore-size sinter layer as a first type of sinter layer and a smaller micropore-size sinter layer as a second type of sinter layer, which are laminated in contact with each other, wherein at least one side of a first type of layer making contact with a side of the other type of sinter layer. The larger micropore-size sinter layers provided in the porous body may be capable of being filled with a solid electrolyte so as to be used for current passages to decrease inner resistance. The smaller micropore-size sinter layers, which increase the capacitance because of high specific surface area of micropore in the layer, increase the total capacitance of the porous body, and reduce the inner resistance.
The laminate of the porous body may include sinter layers, each having a different density from the others. In this anode, the laminate may include high density sinter layers and low-density sinter layers, which are laminated in contact with a high density sinter layers.
Furthermore, in the anode of the present invention, the laminate of the porous body may be formed of sinter layers having a higher specific surface area and sinter layers having a lower specific surface area.
In the invention, the porous body preferably may have corrugated surfaces for connection with a cathode connecting electrode to decrease the contact resistance therewith. For this purpose, a laminate of two types of preforms, each having a different thermal shrinkage ratio during sintering, may be sintered into sintered layers in the porous body.
A process of fabricating such a laminate may comprise sintering a laminate of a plurality of preforms, which are prepared from at least two types of preforms having different properties.
In another process of-the present invention, the plurality of preforms may include the first type of preforms and the second type of performs having preform densities different from each other. The two types of preforms are laminated and the laminate is sintered, to fabricate the porous body.
In a process of producing such a porous body, at least two types of preforms, which are made from valve metal powders having different specific surface areas of the micropores per unit weight, is laminated in a laminate, and sintered into a porous body.
Particularly, the anode for an electrolytic capacitor of the present invention may be produced by sintering a laminate of two types of preforms made from powders of secondary particles having different physical strengths agglomerated from primary particles of a valve metal.
Further, it is preferable that the preforms are made sufficiently thin in the shape of a plate or sheet, and the porous body is joined with a valve metal substrate, or metal foil, so that the valve metal foil is disposed substantially in parallel to the sinter layers formed of the preforms. Alternatively, the valve metal substrate may be disposed substantially perpendicular to the sinter layers.
Further, in the process, the preforms may include plates or sheets formed of valve metal powder. The plates or sheets may be stacked on the substrate of valve metal foil, the laminate being sintered as a whole. In another process, first the porous body may be formed by sintering the laminate bf the preforms, and then the sintered porous body is joined, either in parallel or perpendicularly, with the valve metal substrate.